1. Field of the Invention
This invention relates to the field of current mirrors, and particularly to current mirrors having low headroom requirements.
2. Description of the Related Art
Current mirrors are ubiquitous in analog circuit design. A given current mirror circuit requires a minimum amount of “headroom”—i.e., the supply voltage for the mirror and its driving circuitry must be greater than a particular minimum to ensure proper operation.
For example, the output of a bandgap voltage reference is often used to generate bias currents; one circuit for doing this is shown in FIG. 1a. A temperature-compensated reference voltage Vref, typically ˜1.2 volts, is buffered with an amplifier A1, the output of which drives a ground-referred bipolar transistor 10, with the resulting current mirrored to provide the bias currents 12, 14. If a simple field-effect transistor (FET) mirror 16 is used, the supply voltage (V+) needed for proper operation will have to include the mirror FETs' drain-source voltage (Vds), the base-emitter voltage (Vbe) of bipolar transistor 10, and reference voltage Vref. Thus, the minimum supply voltage required for this circuit is approximately given by:V+(min)=Vref+Vbe+Vds=1.2V+0.6V+1V=2.8V.
For better performance, a cascoded current mirror 18 may be employed, as shown in FIG. 1b. Here, however, an additional drain-source voltage must be accommodated. Therefore, for this arrangement, V+(min) is approximately given by:V+(min)=Vref+Vbe+2*Vds=1.2V+0.6V+2V=3.8V.
However, analog circuitry is increasingly required to operate with supply voltages that are less than those calculated above. As such, there is a need for a current mirror circuit having a low headroom requirement.